High Power Factor and Enhanced Thermoelectric Performance in Sc and Bi Codoped GeTe: Insights into the Hidden Role of Rhombohedral Distortion Degree

Abstract

Modification of crystal symmetry induced by chemical doping or alloying is well known to optimize the charge carrier transport properties in thermoelectric materials. Historically, the altered defect chemistry of the targeted materials, however, has long been neglected or underestimated, which may, in turn, favorably or adversely affect the thermoelectric properties. Herein, using a case study of thermoelectric GeTe, first, the hidden role of rhombohedral distortion degree on the Ge‐vacancy formation energy is theoretically unraveled, and it is experimentally found that Sc and Bi codoping realize a superior thermoelectric performance. Density functional theory calculations demonstrate that the distorted rhombohedral lattice closer to cubic would unexpectedly induce the significantly decreased formation energy of Ge vacancies, resulting in the spontaneous formation of higher‐concentration Ge vacancies. Sc doping is found to be the most effective dopant to reduce the carrier concentration without obviously affecting the rhombohedral distortion degree, leading to the realization of a record‐high power factor. Benefiting from the suppressed thermal conductivity by further Bi doping, an ultrahigh average ZT ≈1.2 from 300 to 723 K is achieved. These discoveries provide new insights into the relationship between crystal symmetry and defect chemistry, and also promote GeTe materials for future thermoelectric applications.

Versatile Vanadium Doping Induces High Thermoelectric Performance in GeTe via Band Alignment and Structural Modulation

“…[23,24] Considering the degenerated transport properties and optimization of carrier concentration, ZT values of GeTe can be greatly enhanced through rational doping methods in the low-and mid-entropy regime. [25] To achieve this goal, various dopants that replace the Ge site have been applied to alter the GeTe band structure (e.g., Sc, [26] Ti, [27] Zr, [28] Mn, [11,29] Cr, [30,31] Ag, [32] In, [33] etc. ), with a peak ZT value above 1.0 frequently achieved, as summarized in Figure 1c.…”

“…In order to address the inherently high hole concentration in GeTe, aliovalent elements (e.g., Bi or Sb) have been frequently alloyed into GeTe with reproducible peak ZT values exceeding 1.5 or even 2.0. [26,[33][34][35][36][37][38] Recently, doping of transition metals with partially filled d orbitals (e.g., Ti, Mn, and Cr) have been reported to have a significant impact on the structures and subsequently the ZT of GeTe-based materials, [11,29,[39][40][41][42] providing new insights when screening cutting-edge thermoelectric candidates. For instance, Ti and Mn doping can reduce the c/a ratio to obtain cubic GeTe at room temperature, which promotes the band degeneracy and band convergence for a higher effective mass and ZT; [11,29,42] Cd and Cu can form various lattice defects acting as extrinsic phonon scattering sources, for example, planar vacancies, point defects, and nanoprecipitates (NP), to push κ l toward the amorphous limit.…”

“…Very recently, yttrium (Y) was reported to sharply reduce the carrier concentration and lightly increase the rhombohedral angle, leading to an enhancement of power factor and a high average ZT in GeTe, which favors the power-generation applications. [43] Although transition [11,26,27,30,32,[46][47][48][49] d) Calculated electron localization function for pristine CGeTe and V-doped CGeTe, respectively. e, f) Band structure and DOS calculated by DFT for pristine CGeTe and V-doped CGeTe, in which valence band maximum is pinned at 0 eV.…”

“…b) HAADF STEM image of R-GeTe, viewing along the [011] direction. c) Comparison of ZTs of GeTe-based materials with single dopants [11,26,27,30,32,[46][47][48][49]. d) Calculated electron localization function for pristine CGeTe and V-doped CGeTe, respectively.…”

A material catalogue with glass-like thermal conductivity mediated by crystallographic occupancy for thermoelectric application

“…[3][4][5] The dimensionless thermoelectric figure of merit (ZT), defined as ZT = (S 2 σ/κtot)T, dominates the conversion efficiency, where S, σ, κtot, and T are Seebeck coefficient, electrical conductivity, total thermal conductivity (including lattice thermal conductivity κlat and electronic thermal conductivity κele) and absolute temperature, respectively. κele usually follows the Wiedemann-Franz law, linearly proportional to σ and T. Considering the intertwined or contradicted thermoelectric parameters (S, σ, and κele), 6 optimizing the charge carrier concentration [7][8][9] or suppressing κlat through nano-microstructural engineering [10][11][12][13][14][15] is the common method to increase ZT. Please do not adjust margins Please do not adjust margins Alternatively, discovering materials with the intrinsically low κlat is another important route for achieving high ZT in thermoelectrics.…”